Two-filament arc discharge lamp having alternating discharge spots thereon



3,3 78,724 TEHNATING A ril 16, 1968 DI SCHAR GE SPOTS THEREON 2 Sheets-Sheet 1 Filed Aug. 3, 1965 2 M 3 r W mm 6 0 R w o w F R w L P F M Z M 0 0 0 U m m w m 6 hfiwwafiwmwmwk .Qmi 3Q M J k 3 4 w I 6 3 6 w A 4 W F l J fluxm w 2 6 l), m m 7 a 3 9 F w 5 Q 2 4+w md m M m 2 0 H m m m m 3 5% U RAA NAM a k may mm m A M 7 M N K A mm 0 L, o m w IIYIM 8 1 United States Patent 3,378,724 TWO-FILAMEN T ARC DISCHARGE LAMP HAVING ALTERNATING DISCHARGE SPOTS THEREON Isao Kaneda and lime Kanayama, Otsn Shiga, Yoshiro Kitagawa, Kusatsu Shiga, and Hiroshi Kita, Otsu Shiga, Japan, assignors to New Nippon Electric Company Ltd., Osaka, Japan Filed Aug. 3, 1965, Ser. No. 476,895 11 Claims. (Cl. BIS-97) ABSTRACT OF THE DISCLOSURE To provide a high output apparatus for a discharge lamp without reducing the lamp life and eificiency as compared with that of conventional operation, it is incorporated into a circuit which causes alternating discharge spots to be formed on the filaments. This is achieved by applying a different frequency across the filaments than the frequency applied between the filaments.

This invention relates to devices employing filaments which are operated at elevated temperatures, and more particularly to circuit for devices such as fluorescent lamps and the like.

It is an object of this invention to provide improved lighting circuits for discharge lamps, and more particularly improved circuits in which low pressure mercury vapor lamps such as fluorescent lamps and the like can be operated at high outputs with high elficiencyor with prolonged life. It is also an object of this invention to provide improvements in circuit and lamp efiiciency and to provide improved compact ballast.

It is known that fluorescent lamps can be operated at high outputs when lamp current is increased over rating. However, in such operation, the are spot of a cathode filament is excessively heated resulting in shortened life and reduced lamp efliciency. For example, a conventional 40 w. lamp having a tube 1.5 inches in diameter and 48 inches long, when operated at 200 watts, has its efliciency reduced to 34% of its optimum value.

Another well known method of increasing lamp output is the so-called double spots method. The double spots method is disclosed, for example, in T. Kobayashi Patents 2,802,143, 2,906,923, 3,047,712, and 3,114,076. In the circuits disclosed an impedance connected in series to lamp filaments is divided or the phases of lamp voltage and filament voltage are shifted by 90 degrees so that two arc spots are formed on a single filament thus increasing lamp current to substantially twice that of a conventional lamp without the defects of short life and low efiiciency.

The present invention has, in common with the above described double spots system, the feature that a lamp can draw increased current and operate at high output with high efiiciency, while the defects of short life and low efllciency are eliminated.

In the invention, however, a single are spot is alternately formed at either end portion of each of the oppositely provided electrodes of a lamp. Therefore, at a given moment, there is theoretically formed only one are spot. However, when the alternating are spot forming cycle is short enough in comparison with the heat capacity or inertia of the filament, there appears an apparent plurality of arc spots at the points adjacent the terminals of the filament. Thus the increased lamp current is distributed to a plurality of spots.

The manner of forming an arc spot in a conventional fluorescent lamp is as follows: When the cathodes are assumed to be made uniform, there appears one spot on the cathode and on the anode between which spots the 3,378,724 Patented Apr. 16, 1968 potential difference formed by the voltage supply is the largest. When the electric source is alternating, this spot is formed and extinguished on these two filament electrodes according to the cycle of the source voltage.

In accordance with the invention, when a voltage having a frequency, for example, a high frequency voltage, which is different from the primary supply voltage is additionally applied, this high frequency voltage superposed on the primary source voltage causes two are spots on each cathode filament at the end portions thereof, which in reality are constituted by a single spot shifting between the points on the electrodes between which there is the largest potential difference. This: shift is based on the difference of the frequencies of the primary source voltage and the additionally applied high frequency voltage.

The invention will next be described hereinafter in greater detail with reference to the acompanying drawing in which:

FIG. 1 is a schematic diagram of a circuit illustrating the principles of the invention;

FIG. 2 is a graph showing the relationship between frequency and temperature in a fluorescent lamp;

FIG. 3 is a graph showing the relationship between lamp current and temperature in a fluorescent lamp;

FIG. 4 illustrates an embodiment of the invention in which shifting arc spots are employed;

FIG. 5 illustrates another embodiment employing an auxiliary starting device;

FIG. 6 is a schematic diagram of still another embodiment of the invention;

FIG. 7 is a chart illustrating a fluorescent lamp waveform;

FIG. 8 illustrates another embodiment of the invention;

FIG. 9 is a schematic diagram of still another embodiment; and

FIG. 10 illustrates lamp voltage waveforms;

In FIG. 1, which illustrates the principles of this invention as noted above, points 1, 2, 3 and 4 are the points at which an are spot is most likely to be formed. Elements 5 and '6 are alternating current sources which heat the filament electrodes 9 and 10. Component 7 is a current limiting resistor, and element 8 is a direct-current source for operating the lamp 11 which is a fluorescent lamp.

When the lamp is operated in this circuit, the direction of the lamp current is either from points 1 to 2 or 3 to 4. However, as mentioned above, there are provided the alternating current sources 5 and 6, these sources having a frequency (7). Because of the voltage oscillation between points 1-2 and 3-4 due to these A.C. sources, a discharge path is formed between one of points 1 and 2, and one of points 3 and 4.

When the lamp is operated by an A.C. source 13, instead of the DC. source 8, the direction of the discharge. path is alternatingly changed and the resistor 7 is replaced by a current limiting impedance 12.

In the above-described arrangement, if the filament voltage is kept above a given value, a method of alternatingly forming an are spot at a plurality of points on the respective electrodes, but not forming a plurality of arc spots on any one of the electrodes at a given time, is provided. If this oscillation cycle is suitably selected with respect to the heat inertia of the filament at the are spot, the lamp current can be distributed through two spots. This can be achieved by frequency application according to the invention.

More particularly, if on the supply voltage, an additional voltage of different frequency such as a high frequency voltage (for example, from the sources 5 and 6 in pressed as:

lf2 f1l When the difference of frequencies is small, only one are spot is seen and the movement of this spot from one end to the other along the filament can be followed with the naked eye. When the difference increases to a certain order (|f f [=60 c./s.) two are spots appear to remain on the filament.

FIG. 2 shows the relationship between the frequency and the red-heat temperature in a 40 w. rapid-start type fluorescent lamp. In this graph, the abscissa shows the frequency (f of the electrode heating voltage and the vertical axis shows the temperature of a red hot spot. From this graph, it is seen that the current can be increased in proportion to the increase of a cycle of oscillation of the are spot. Therefore, when l is larger than the frequency of the primary source, a result superior to that of the so-called double spot method can be expected. In other words, the change of the temperature of a spot on the filament is much slower than the movement of the spot due to heat capacity of the filament. Thus, by selecting a suitable value for |f ;f the temperature of the red hot point is evenly distributed and the maximum temperature of the filament is reduced for the same current. Since the evaporation of the electron emissive material is related to the temperature of the electrode, the permissive range of the value of current can be broadened.

The range of the frequency and voltage by which the arc spot is oscillated is, within such values, quite easily obtained. For example, a high frequency filament heating source was experimentally found to be sufiicient at about 0.8 v., this value being a very small fraction of the filament voltage (about 4 v.) used during operation of a lamp at 60 c./s.

Although the arc spots are seen as being divided into two points, the current on the filament lead-in wires being switched does not become zero, according to the difference of the two frequencies after oscillation is achieved.

FIG. 3 shows the relationship of lamp current and the temperature of the red hot spot. In this graph, the curve a is that of a conventional 40 w. rapid-start type fiuorescent lamp with a rapid-start ballast. This lamp is operated at 40 watts loading, the rated lamp current is 0.435 a., and the temperature raises to about 1180 C. because, in this case, the phases of the filament heating source and the lamp voltage are approximately the same.

However, if the filament is heated by a supply of a different frequency, such as a 180 c./s., 30 v. supply, two are spots are seen on each filament. The results are shown by the curves b and c. At the same are spot temperature, the lamp can draw a current of 0.8 a. (in case of the curve b) while operating. It therefore follows that if the are spot is oscillated, lamp life can be extended for a given lamp current.

In the well known double spots method, an are spot is divided with a division of current which runs into the two spots. This is achieved by balancing the impedance elements connected to each filament terminal of an electrode. However, in actuality, the temperatures of these two are spots are not found to be the same, even though not theoretically well explained.

According to this invention, this difiiculty of balancing the temperature of spots is eliminated. Additionally, as far as an are spot is concerned, it can be understood in the same way as a high frequency operation in which electrode loss is rather small. That means the temperature of an are spot is not as high as that of usual frequency operation. That is, according to formula (1) above, the electrode loss is reduced. Indeed, excessive filament voltage increases the electrode loss, and there is an optimum value of filament voltage.

In the example shown in FIG. 8 (as will hereinafter be described) the filament voltage Vf=1.5 V. was found by a monochrometer to minimize the evaporation of the emissive material for a 40 W. fluorescent lamp. The ballast construction according to this invention can supply a cathode voltage of 4 V. and 60 c./s. when starting a lamp and 1.5 V. and 180 c./s. after starting. Therefore, it extends the upper limit of the lamp current in comparison with a conventional ballast with which 4 V. 60 c./s. remains on the filament after starting or a ballast with which the filament voltages are canceled after starting. This can be explained as follows: when the spot is considered, the heating and cooling of the spot is the same as a high frequency operation; therefore, the electrode loss is small; another reason is that an exchange of power between two sources of different frequencies does not seem to take place.

Referring next to the embodiment illustrated in FIG. 4, element 101 is a fluorescent lamp having the oppositely provided electrodes 102 and 103. Elements 104, 105, 106 and 107 are the terminals of the electrodes 102 and 103. Component 108 is a primary A.C. voltage source. Component 109 is a leakage ballast comprising a primary coll 110 and a secondary coil 111. Part 112 is an additional high-frequency source for heating the electrodes. Part 113 is a transformer comprising a primary coil 114 and secondary coils 115 and 116. Source 112 has a different frequency from that of the primary source 108. For example, when the source 108 is provided with a commercially available frequency, the source 112 may have a high frequency such as about 2 kc. The secondary coils 115 and 116 of the transformer 1.13 are connected to the electrodes 102 and 103 respectively.

Electrodes 102 and 103 are heated by a high-frequency voltage induced in the secondary coils 11S and 116 of the transformer 113. Consequently when the voltage induced in the secondary coil 111 of the leakage ballast 109 is applied to the electrodes 102 and 103, the lamp starts to operate. After that, discharge is stably maintained, as well known, by the inductance of the secondary coil 111.

During operation of the lamp, the electrical potential of the electrodes 102 and 103 is alternated according to the cycle of the primary source 108 as well known. Moreover, according to this invention, the potential of the point adjacent each terminal 104, 105, 106, 107 is alternated according to the cycle of the source 112 during every half cycle of the primary voltage source.

Assuming the polarity indicated in FIG. 4 in a half cycle of the high-frequency source 112 during a half cycle of the primary source 108, the arc spot is formed on the cathode 103 adjacent its terminal 106. The lamp current flows from the terminal 104 to the terminal 106. In the following half cycle of the source 112, the polarities of the terminals 104, 105 and 106, 107 are reversed, while the polarity of the voltage induced by the secondary coil 111 of the leakage ballast 109 by the source 108 remains unchanged. This time, an arc spot is formed at the other end of the filament of the cathode 103. The lamp current now flows from the terminal 105 to the terminal 107 and the prior arc spot extinguishes. Indeed, the same alternation of the spot takes place in the subsequent half cycle of the source 108, except that this time the direction of lamp current is reversed.

An are spot, as described heretofore, is formed at two points of a cathode filament alternatingly according to the difference of the frequencies of two electric sources. However, due to the heat capacity of the cathode filarent, when alternation of the spots is rapid enough, there effectively result two apparently simultaneous arc spots on a single cathode filament. Therefore, the lamp can draw more lamp current. than with a conventional circuit thus achieving a more efficient and higher-output operation than is possible with a lamp operation in a conventional circuit with increased current.

An experiment on the above was conducted with a 40 w. fluorescent lamp. As a primary electric source there was employed 200 v., 60 c./s.; and as a cathode heating or potential alternating electric source 4 w., 2 kc. inverter were used. A lamp current of 910 ma. Was obtained, while the lamp current of a conventional fluorescent lamp with a single are spot drew only 435 ma. This is about a twofold increase. Outputs of about two times as large as a lamp with a conventional circuit were obtained. (In this experiment, the circuit shown in FIG. 4 was used.)

Another embodiment, which is particularly advantageous for starting a lamp is shown in FIG. 5. In this circuit, a high-voltage coil 117 is added to the filament transformer 113. This coil 117 is so connected as to apply high voltage to the electrodes 102 and 103 through a capacitor 118 to start the lamp. On the other hand, the primary coil 110 (FIG. 4) of the leakage transformer 109 which develops the voltage of the primary source is eliminated. This embodiment provides a rapid start circuit. Additionally the capacitor 118 and the impedance element 111 prevent the mutual interference of the source 108 and source 112 while the lamp is operating.

By the circuits shown in FIGS. 4 and 5, two are spots are formed on each cathode filament. It is, however, also possible to use an inverter instead of the high-frequency filament heating source 112, so that the inverter can be operated by the source 108.

Further, although it has been described that the source 108 is a commercially available one, it is possible to have a source 108 with a higher frequency than that of the source 112. Furthermore, when many lamps are operated at once, it is advantageous to make the high frequency source common to all lamps.

As described, this invention eliminates the disadvantage of the short life of fluorescent lamps at high output operation by forming a plurality of arc spots. Since the distribution of the arc spots is based on oscillation, the spots are more stabilized than with the known double spots operation. Also, this invention makes it possible to provide a lighting system with fluorescent lamps at a much reduced cost.

Additionally, in these circuits, it is possible to provide taps a and b on one of the secondary coils 115 and 116 of the transformer 113 and to supply lamp power therefrom. In this case the magnitude of the alternating arc spots can be made uniform by selecting the positions of the taps. When a suitable device is used as the source 112, the circuit can be provided at less cost than the conventional double spot circuit. Further, if the starting voltage is derived from the high-frequency voltage which alternates the spot, the core of the leakage transformer can be made smaller and the turns of the coil can be reduced.

Further to the above, it is Well known that lamp voltage has a substantially rectangular waveform. This rectangular wave contains odd number high harmonics. According to another embodiment of the invention, if a suitable device is employed, the cathodes of a lamp can be effectively heated and the spot alternated by these high harmonics.

In FIG. 6, elements 2.01 and 202 are the terminals of an electric source of commercially available frequency. Element 203 is an impedance element for the stabilization of lamp operation. Part 204 is a fluorescent lamp with electrodes 205 and 206. Next, in accordance with this invention, there is provided a high pass filter 207 between each of the terminals of the lamp cathodes. Also, the secondary coils 209 and 210 of cathode heating transformer 208 are connected to the cathodes 205 and 206 respectively. The primary coil 2.11 of the transformer 208 is so arranged as to be operated by the outputs of the filter 207.

In the above described arrangement, if the discharge lamp is started by any conventional means, the waveform of the lamp voltage becomes rectangular as shown in FIG.

. 7. The development of the rectangular Waveform is a well known characteristic of discharge tubes according to which the voltage drop across a tube follows the applied voltage until discharge occurs after which the voltage drop as shown in FIG. 7 maintains an almost constant level until the discharge is extinguished. This rectangular voltage is applied to the filter 207. As is well known, a rectangular wave contains many high harmonics. The output of the filter 207 is mainly constituted by voltages of third high harmonics. This voltage is applied to the cathodes 205 and 206 through the transformer 208,. so that the cathodes 205 and 206 are heated by a voltage having a frequency of three times that of the discharge lamp current.

Therefore, the electric potential at either end of each filament is alternatingly changed during each half cycle of lamp current. With this alternation of the potential, an arc spot is alternatingly formed at two points on each filament which substantially appear as two spots. Thus, the lamp can draw increased current Without excessive heating of the arc spot.

FIG. 8 shows another embodiment of this invention. In this embodiment, each primary coil 2223 and 224 of the transformers 221 and 222, which are connected in series with each other, is disposed between the electrodes 20 5 and 206 of lamp 204. A capacitor 225 is connected in parallel with the primary coil 224- of the transformer 222.

According to this arrangement, a voltage having a higher frequency than the voltage applied across the tube is applied to the primary coil 223 of the transformer 221 due to the filtering of the rectangular Waveform discussed above relative to FIG. 7. More particularly, pairs of secondary coils 226 and 227, and 22 8 and 229, of the transformers 221 and 222, the coils 226 and 228 and the coils 227 and 229 are connected respectively in series, with opposite polarities, to each other and are connected to apply voltage to cathodes 205 and 206 respectively. Thus, each cathode is heated by the high frequency components of the discharge rectangular-Wave voltage. Therefore, a function which is the same as in the embodiment shown in FIG. 6 is achieved.

The terminal voltage of the lamp before it starts to operate is a sine curve. In the embodiment shown in FIG. 8, the secondary coils of the transformers 221 and 222 are arranged in such a manner that the basic wave .in the rectangular Wave and the basic wave from the main source cancel each other. With this arrangement therefore, the cathodes cannot be heated and consequently the lamp cannot start to operate unless a suitable countermeasure is taken. In order to overcome this problem, a magnetic material having good hysteresis characteristics is used for the core of the transformer 221. A high voltage with which the electrodes are heated is induced only in the secondary coils 228 and 229 of the transformer 222 because, when the source is applied, a voltage equal to the source voltage is applied to either end of the lamp and the core of the transformer 2211 saturates immediately. Almost all source voltage is applied to the primary c oil 22 4 of the transformer 222, and only slight voltage is induced in the secondary coils 226 and 227. The ballast 203 and the capacitor 225 are connected in series so that, at this time, the voltage to be impressed on the terminals of the lamp is developed, resulting in a good star-ting of the lamp. After the lamp is started, the lamp voltage decreases and the core of the transformer 221 is no longer saturated. Then it functions as described above.

In the above arrangement, if this circuit is connected to a source of 60 c./s., 200 v., the capacitor 225 induces a con'densor voltage of about 270 v. due to the series connection of the ballast 20 3, capacitor 225' and the primary coil 223 of the transformer 221. This condenser voltage,

7 which is impressed on the filaments 205 and 206, is well above the voltage required (230 v.) by the lamp to start.

The filaments 205 and 206 are, at this time, heated by the rated preheating power, so that the lamp can start. Additionally, this arrangement eliminates the leakage tnansformer to develop the voltage and reduce the cost. Also, the ballast loss of a conventional 4-0 W. one-lamp ballast is about 18.2 W. and the efiiciency of the lighting system (including ballast and lamp) is about 51 lm./w., while according to this invention, the efliciency is kept at more than 51 lm./W. at 60 W. operation of the same lamp. This is explained by reduction of circuit loss (except in the lamp). Furthermore, the. reduction of spatter at starting and during operation can be observed in the combination of a lamp and circuit according to this invention.

As described heretofore, the last two embodiments of the invention utilize high harmonics contained in the rectangular wave of the lamp voltage, so that the cathode heating voltages or spot alternating voltages of frequency different from that of the lamp operating voltage are easily obtained. In a test conducted relative to these embodiments, the following data were obtained:

8 is closed by the switch 306, the filaments 302 and 302 are heated by the power necessary to preheat them through the transformer 310. The secondary voltage of the transformer 304 is impressed on the filaments 302 and 302; consequently, the lamp is started and at the same time switch 306 is turned off.

After a lamp is started, the lamp voltage takes theform as shown in FIG. 10(A) which is the well known rectangular wave indicated hereinabove. In this wave, there are contained high frequencies. These high frequencies are derived from the potential oscillation of the'anode. Indeed the frequencies of this oscillation are much higher (about 2 kc./ s. than that of the source 303.

In operation if the electrode 302 is the anode, the-secondary coil 308 is energized by the inherent potential variations which will take place in the filament 302 due to minor current fluctuations. A voltage is thereby induced in the secondary coil 309 which is electromagnetically connected to this coil 308. The potential of two points of the filament 302 is therefore oscillated by the voltage induced in this coil 309 since this latter voltage is of a higher frequency than the voltage applied across the tube. At

Comparative Data FLR: Rapid start fluorescent lamp. Wtd=Power consumption of lamp. Wp=Power consumption of lighting system. lm./Wp=Efficieney of lighting system.

Ib= Lamp current.

Etd=Lamp voltage.

The experiment was conducted with the circuit shown in Fig. 8. A.

200 v. Capacitor 1.5 pf.

It has been proposed above the operate fluorescent lamps and the like in such a manner as to form an arc spot alternatingly at two different points on a cathode filament so that increased supply current can be distributed between two spots without excessive heating of any are spot while achieving high output operation.

It is advantageous, in the above method, that the arc alternating cycle on the cathode filament be short enough that the supply current can be desirably distributed. Therefore it is necessary to connect the cathode filament to a high frequency supply source.

In a fluorescent lamp or the like, an are spot takes on aspects when a filament is functioning as a cathode which are different from when the filament is functioning as an anode. When the electrode is positive or functioning as an anode, the electrons run into the filament electrode substantially all over the surface thereof. On the contrary, ion flow is concentrated to one point of the cathode filament. Therefore, in order to operate a lamp at high outputs, it is not always necessary to oscillate the potential of a plurality of points on the anode electrode and it is enough to oscillate only the points of the cathode.

In accordance with the following embodiments of the invention, it is an object to operate lamps at high outputs in such a manner that the oscillation of the potenial disribution of the anode electrode (anode oscillation) is utilized to oscillate the are spot on the cathode filament.

One such embodiment of this invention is shown in FIG. 9 wherein fluorescent lamp 301 has oppositely provided filaments 302 and 302. Alternating current source 303 is used for operating the lamp. Ballast 304 is, for example, a leakage transformer. Impedance element 305 is used for limiting current to the lamp. Element 306 is a switch for starting the lamp. In this embodiment, a transformer 3210 having a primary coil 307 and secondary icoils 8 and 309 is provided. The primary coil is connected to the source 303 though the switch 306 and the secondary coils 303 and 309 are connected to the filaments 302 and 302 respectively.

In the above described arrangement, when the circuit C. source c./s.

this moment, the filament 302 is functioning as a cathode, so that an are spot is alternatingly formed on two points of the cathode filament. As a result, the lamp is operated at high output.

When the filament 302' functions as an anode,'the secondary coil 309 is energized by the potential oscillation of the filament 302'. The induced voltage in the secondary coil 308 is impressed on the cathode 302.

In order to cancel the variation of magnetic flux being induced in the core of the transformer 308 or 309 according to the frequency of the source 303, which is due to the secondary current of the transformer 304 flowing to the secondary coils 308 and 309, it is necessary to connect to or wind the secondary coils 308 and 309, as shown in the drawing, about the core of the transformer 310. If not so connected, the spots on the filaments 302 and 302' are oscillated according to the source frequency and not according to the anode oscillation.

The invention has been explained with fluorescent lamps, but the circuits are also applicable to all types of low pressure mercury vapor discharge lamps for illumination, sterilization, photocopy uses, and so forth.

There will now be obvious to thoseskilled in the art many variations of the above apparatus and techniques. The variations will come within the scope of the invention if defined by the following claims.

What we claim is:

1. Apparatus comprising a discharge lamp including at least one filament and at least one first means adapted to function as an anode, cooperating with said filament, second means to apply an AC. voltage between said filament and said first means to pass a current through said lamp, and third means to cause alternating arc spots to be formed on said filament through which said current passes, said first means being a filament spaced from the first said filament, said filaments interchangeably functioning as anodes and cathodes depending on the voltage supplied by said second means, said third means supplying power to said filaments and causing alternating arc spots to be formed on the first said filament, said second and third means including sources of AC. voltages of different frequencies.

2. Apparatus as claimed in claim 1, wherein said third means includes filament power supplies coupled across respective of said filaments and said second means includes a voltage source coupled between said filaments.

3. Apparatus as claimed in claim 1, wherein said third means comprises a filament power source and a transformer coupled thereto, said transformer including two secondary windings respectively coupled across said filaments.

4. Apparatus as claimed in claim 1, wherein said third means comprises a filament power source, a transformer coupled to said source and including two secondary windings respectively coupled across said filaments, said transformer further including a high voltage winding and capacitor coupled in series between said secondary windings to provide a rapid start circuit.

5. Apparatus comprising a discharge lamp including at least one filament and at least one first means adapted to function as an anode cooperating with said filament, second means to apply an AC. voltage between said filament and said first means to pass a current through said lamp, and third means to cause alternating arc spots to he formed on said filament through which said current passes, said first means being a filament spaced from the first said filament, said filaments interchangeably functioning as anodes and cathodes depending on the voltage supplied by said second means, said third means supplying power to said filaments and causing alternating are spots to be formed on the first said filament, the voltage waveform which results between said filaments being rectangular said third means comprising filter means coupled between said filaments to derive relatively high frequency components from said waveform and to apply such components across said filaments.

6. Apparatus as claimed in claim 5, wherein said filter means is a high pass filter adapted for passing third harmonies, said third means further comprising a transformer coupled to said filter and including two secondary windings respectively coupled across said filaments.

7. Apparatus as claimed in claim 5, wherein said third means includes two transformers including respective primary windings connected in series between said filaments, said transformers each further including first and second secondary windings, the first secondary windings being connected in series with opposite polarity across one of the filaments, the second secondary windings being connected in series with opposite polarity across the other of the filaments.

8. Apparatus as claimed in claim 7, comprising a capacitor connected across the primary winding of one of said transformers.

9. Apparatus as claimed in claim 8, wherein the other of said transformers includes a core adapted for rapid saturation.

10. Apparatus as claimed in claim 1, wherein said third means causes alternating arc spots only on the filament which is instantaneously functioning as the cathode according to the cycle of the AC. voltage supplied by said second means.

11. Apparatus as claimed in claim 22, wherein said second and third means collectively comprises a source of said AC. voltage, and a switch and a transformer connected in series with said source, said transformer including two electromagnetically coupled secondary windings respectively coupled to said filaments, said source being coupled between said filaments.

References Cited UNITED STATES PATENTS 2,351,270 6/1944 Lemmers 315174 X 2,586,400 2/1952 Waguet 315105 X 2,906,923 9/ 1959 Kobayashi 315- FOREIGN PATENTS 314,088 5/1930 Great Britain.

DAVID J. GALVIN, Primary Examiner. 

